Thermally sensitive controls

ABSTRACT

A thermally sensitive, overheat protection control for a thick film electric heater comprises a bimetallic actuator ( 68 ) arranged for making good thermal contact with the heater, and a moveable electrical contact ( 78 ) for engagement with a contact provided on the heater. The moveable contact ( 78 ) is operatively coupled to the bimetallic actuator ( 68 ) such that in use upon operation of the actuator, it moves out of engagement with the heater contact to interrupt the electrical supply to the heater. The bimetallic actuator ( 68 ) and the moveable contact are mounted on a common generally U-shaped leaf spring ( 66 ) which acts in use resiliently to bias the actuator ( 68 ) into thermal contact with the heater and the moveable contact ( 78 ) into electrical contact therewith. The control also comprises a creep action bimetallic actuator ( 110 ) mounting a contact at one end, the actuator acting as a thermostatic control for the heater.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to thermally sensitive controls forelectric heaters, and more particularly for thick film electric heatersused in liquid heating vessels.

2. Background Information

So called thick film electric heaters comprise an electrically resistiveheating track deposited, for example printed, onto an electricallyinsulating substrate provided typically as a ceramic or an insulatedmetallic support plate. In liquid heating vessels the metallic supportplate may be formed by the base of the vessel itself, or by a platemounted in an opening in the base of the vessel. Examples of suchheaters are shown, for example in WO96/18331 and WO96/17497.

Typically the heating track has a tortuous path terminating in a pair ofterminals through which power is supplied to the track. The power supplyto the track is normally controlled by a thermally sensitive controlwhich acts to open a set of contacts in the control to disconnect thepower supply to the track in the event, for example, that the heateroverheats. Power leads may be soldered on to the terminals and connectedto the control, or as shown in GB-A-2283156 the thermally sensitivecontrol may be provided with sprung contacts which make electricalconnection with the terminals when the control is mounted to the heater.

A recent proposal by the Applicant in GB 2322274 is to use the contactprovided on the heater as one of a pair of switch contacts, the othercontact being provided in a control or control unit mounted against theheater. In the event that the heater overheats, a bimetallic actuatorarranged in good thermal contact with the heater operates to lift thecontrol contact from the heater contact to interrupt the supply ofenergy to the heater, thereby preventing serious overheating of theheater and possible damage to the liquid heating vessel.

In the earlier proposals, the control mounted contact and the bimetallicactuator are separately mounted in the control. This, however, isinefficient in terms of assembly tolerance and costs. The presentinvention seeks to overcome these drawbacks.

DISCLOSURE OF THE INVENTION

From a first aspect, therefore, the invention provides a thermallysensitive control for a thick film electric heater comprising abimetallic actuator arranged for making good thermal contact with theheater, and a movable electrical contact for engagement with a contactprovided on the heater, said movable contact being operatively coupledto the bimetallic actuator such that in use upon operation of theactuator, it moves out of engagement with the heater contact tointerrupt the electrical supply to the heater, said bimetallic actuatorand said movable contact being mounted on common leaf spring means whichact in use resiliently to bias the actuator into thermal contact withthe heater and the movable contact into electrical contact therewith.

Thus in accordance with this aspect of the invention, the need forseparate mountings for the movable contact and actuator is avoided, anda common leaf spring is used both to mount the actuator and contacts andresiliently to bias them respectively into thermal and electricalcontact with the heater. The leaf spring means thus performs a dualfunction, avoiding duplication of parts. Furthermore, the contact andactuator may be preassembled before being mounted in a control unit, forexample, thereby reducing assembly times and costs. It also allowsaccurate positioning of the respective components relative to eachother. Furthermore, the leaf spring will have a relatively low thermalcapacity, thereby reducing the amount of heat drawn out of the actuatorso improving its thermal response.

The invention also extends to such a preassembly, so from a secondaspect, the invention provides an assembly for use in a thermallysensitive control for a thick film electric heater, the assemblycomprising a bimetallic actuator, and a movable electrical contact, saidbimetallic actuator and said contact being mounted on common leaf springmeans which in use act to resiliently to bias the actuator into thermalcontact with the heater and the movable contact into electrical contacttherewith.

The leaf spring means may in a simple embodiment comprise a singlelimbed leaf spring, with the actuator and contact mounted to differentmounting locations on the leaf spring. In the preferred embodiment,however, the leaf spring means may comprise a leaf spring having atleast two connected limbs mounting respectively the actuator andcontact. This is preferred so that the biasing force of the respectivecomponents can be tailored to suit the required conditions by choosingthe appropriate spring force in each limb.

Preferably the actuator is a bimetallic actuator, preferably a snapacting actuator, and most preferably one of the type described inGB1542252.

In the case of such an actuator, the actuator may typically be biasedagainst the heater with a spring force of 25 g, although lower forcesmay be used. A biasing force of 25 g will ensure good thermal contactbetween the actuator and the heater and allow for thermal distortion ofthe heater, but is not so great as to change the operatingcharacteristics of the actuator. The actuator is preferably mounted byits tongue on the leaf spring member, so as to engage the heater aroundits periphery, thereby giving a large area of thermal contact whileavoiding the need for heat sink compound, which is expensive.

The contact force should be sufficient to ensure a good electricalcontact between the movable contact and the heater contact, but not solarge that it cannot be overcome by the actuator. For a pure silvercontact, a contact force of log is adequate for electrical contactpurposes, and easily overcome by the actuator operating force, whichtypically is in the region of 250 g. For silver alloy contacts, a higherspring force would be necessary for a satisfactory electrical contact.

In the preferred embodiment, the leaf spring member is generallyU-shaped, with the contact and actuator mounted on the respective sidelimbs of the U. The limb mounting the contact member is preferablyarranged so as to overlap a movable portion of the actuator such thatwhen the actuator moves, it moves the contact mounting leg to move thecontact. The contact carrying limb may have a folded up tab for engagingthe actuator, thereby minimising the movement of the actuator requiredto lift the contact, and obviating the need for a separate push rod orthe like to transmit the movement of the actuator to the movablecontact, thereby reducing material and assembly costs.

The leaf spring member, and more particularly the base limb of the Ushaped spring member may be formed with means to mount it in a controlhousing. For example, it may be provided with a resilient bow forfitting in a complementary slot in a control housing.

The leaf spring member is preferably also provided with a terminal, forexample a spade terminal to allow its easy connection to an externalpower supply, for example a connector provided on the end of a wire of apower cable.

The leaf spring member can, as has been said above, be mounted in acontrol housing itself mounted to the heater or the liquid heatingvessel. In other embodiments, however, it may be mounted directly, eg bybonding, to the heater itself, or by rivetting to a separate terminalpiece.

The invention also extends to a combination of a control or assembly ofthe invention in combination with a thick film heater.

As mentioned above, having a separately mounted spring contact andbimetallic actuator does have the disadvantage of being inefficient fromthe point of view of assembly tolerances and costs, these disadvantagesmay be mitigated, to some extent, by having the separate componentscommonly mounted. For example, the respective components may be commonlymounted over one or more mounting posts so that they may be secured bycommon retaining means e.g. screws, nuts, rivets and so on, or, forexample push fitted into a common mounting location.

Thus, compared to the embodiment described earlier, the connecting limbof the common leaf spring means may be substituted by two separatemounting limbs which engage common mounting means. The mounting limbsmay conveniently overlap.

From a further broad aspect, therefore, there is provided a thermallysensitive control for a thick film electric heater comprising abimetallic actuator arranged for making good thermal contact with theheater, and a movable electrical contact for engagement with an contactprovided on the heater, said movable contact being operatively coupledto the bimetallic actuator such that in use upon operation of theactuator, it moves out of engagement with the heater contact tointerrupt the electrical supply to the heater, said bimetallic actuatorand said movable contact being mounted on commonly mounted leaf springmeans which act in use resiliently to bias the actuator into thermalcontact with the heater and the movable contact into electrical contacttherewith.

As in the earlier embodiment discussed, the leaf springs may be mountedin a control housing or directly to the heater itself.

The control of the invention may be most advantageously employed as anoverheat protector for a thick film heater, operating when the heateroverheats, for example in the event that the heater vessel boils dry oris switched on dry. However, it may also be used as a boiling control,in which case the actuator is positioned at a part of the heater whichwill rise in temperature locally when liquid in the vessel boils, e.garrangements as described in GB2283155 or WO 97/04694.

The control described may be employed in vessels which will only heatliquid to boiling, or in thermostatically controlled vessels. If, forexample, the vessel is required only to boil water, an overheatprotector in accordance with the invention may be arranged in one poleof the supply to the heater and the other pole of the supply may beconnected directly to another terminal of the heater. This may mosteasily be effected through a leaf spring mounted contact biased intocontact with a terminal of the heater track. However, if athermostatically controlled vessel is required a thermostatic controlcan be incorporated in the other pole of the supply.

The invention also seeks to provide a simple thermostatic control for aliquid heating vessel having a thick film heater, and so from a furtheraspect, the invention provides a thermostatic control for a liquidheating vessel having a thick film electric heater comprising a currentcarrying creep action bimetallic actuator mounting an electrical contactwhich in use makes contact with a contact provided on the heater, thearrangement being such that in use as liquid in the vessel is heated,the actuator heats and deflects such that at or around a desiredtemperature it will deflect to such an extent as to lift the actuatormounted contact from the heater contact to interrupt the power supply tothe heater.

With such an arrangement, the actuator will initially bias the contactinto contact with the heater contact and as it heats, through acombination of the current flow through the actuator itself and heatreceived from the heater, it will move so as initially to reduce thecontact pressure and eventually, at or around the desired liquidtemperature, open the contacts, thereby interrupting the electricalsupply to the heater. As liquid in the vessel cools, so too will thebimetallic actuator, and it will eventually deflect back towards itsinitial configuration so as once again to close the contacts andreconnect the electrical supply to the heater thereby allowing theliquid in the vessel to be reheated. This cycle will continue tomaintain the liquid at a desired given temperature.

Preferably, means are provided for varying the operating temperature ofthe control. This is achieved in the preferred embodiment by providingmeans for varying the biasing force of the actuator towards the heater,and thus the temperature at which deflection of the actuator will besufficient to open the contacts. Such biasing means preferably comprisesa spring acting on the bimetallic actuator and means for varying theforce exerted by the spring on the actuator. Most preferably, the springmeans comprises a leaf spring and preferably the means for varying thespring force comprises a cam, most preferably a rotary cam, acting todeflect the spring from an initial position.

The thermostatic control may also act as an on/off switch for the vesselin that in one position of the control cam, the actuator mounted contactis spaced away from the corresponding heater contact. Movement of thecam from this position then moves the bimetallic actuator so as to closethe contacts, the spring force applied to the actuator via the leafspring then determining the opening temperature.

As in the control discussed earlier, the bimetallic actuator ispreferably provided with means to mount it in a control housing and alsowith terminals means to allow easy connection to a wire of a power cord.

Preferably the various control components described above are allmounted in a tray-like control housing which can then be mounted to thebottom of the liquid heating vessel so as to make electrical and thermalconnection with the heater at the same time.

It will be appreciated that the invention also extends to a thick filmheater in combination with a control of the invention and also to aliquid heating apparatus, for example a liquid heating vessel orcontainer comprising such a combination.

As was stated earlier, the spring mounted arrangement according to thefirst aspect of the invention may be used as a dry switch on protector,as a boiling control, and/or in conjunction with a thermostatic control.

It would of course be possible to use the spring mountedactuator/contact arrangement as a simple thermostatic control, bychoosing an appropriate operating temperature for the actuator. Withsuch an arrangement, as the actuator heats and cools, it operatesalternately to raise and lower the contact from the heater, therebycycling the power to the heater.

Thus, an arrangement may be envisaged in which, in the variablethermostatic embodiment described above, the variable thermostat isreplaced with a spring mounted actuator as described earlier.

Where a thermostatic control is not required, it may still be desirableto provide a double level of overheat protection for the heater. Thiscan easily be achieved by using two spring mounted actuator assembliesin accordance with the invention, set to operate at a suitabletemperature.

The actuators are preferably set to operate at generally the sametemperature, within normal manufacturing tolerances, i.e. within 25°-30°C. of one another.

Preferably the assemblies are arranged in the respective poles of thesupply to the heater.

The actuator/contact assembly or assemblies may be adapted and arrangedso as to reset automatically when the actuator temperature falls below apredetermined value, thereby continually cycling the power supply to theheater. However, the assemblies may also be adapted so as to requiremanual resetting after operation. To this end, operation of the actuatormay act to trip a spring loaded lever which maintains the spring mountedcontact away from the heater contact until it is manually reset by auser. For example, the contact mounting spring or a member coupledthereto may act on the trip lever so as to trip it. Alternatively,however, the actuator may first act on the trip lever, movement of thelever then acting to move the contact.

In the double actuator/contact assembly arrangements described above,the actuators may be arranged so that operation of one or other of theactuators operates the trip lever, thereby ensuring that both contactsare lifted from the heater.

The trip lever may be coupled to an operating knob or the like foroperation by a user, and it can act as an on/off switch for the heater.

The trip lever may be associated with a steam sensitive actuator which,upon boiling of liquid in the vessel, acts to trip the trip lever andopen the contact (s). An example of the association of steam sensitiveactuator with a trip lever is disclosed in WO95/34187 to which furtherreference may be made for further detail.

It is usual in electric liquid heating appliances to provide a neon orother power indicator for the purpose of showing that the heater of thevessel is being energised. In the above embodiment where switchingoccurs effectively directly onto the track, it is difficult to provide apower supply to the indicator which will indicate whether the heater isenergised. This can, however, be overcome by making the neon connectionsto the heater whereby should power supply to the heater be discontinued,the neon or other indicator will not be illuminated.

This could be effected by making hard wired connections to appropriatecontact regions of the heater but in a preferred embodiment resilientindicator contacts are provided on the control unit which make electriccontact with an appropriate part of the heater when the control ismounted thereto.

Preferably the contacts provided on the control comprise leaf springs.

Most conveniently, the indicator contact on the heater is provided atsubstantially the same potential as the main switching contact.Accordingly the heater indicator contact may be connected electricallyto the heater switch contact by a low resistance material such assilver. Most easily, a double contact arrangement can be provided ateach end of the heater track, whereby the indicator, when operational,is connected in parallel with the heater track.

From a further broad aspect, therefore, the present invention provides athick film heater having a thick film track with terminal means whichare connected, in use, to a power supply indicator.

From a yet further aspect, the invention provides a thermally sensitivecontrol unit for a thick film heater, said control unit having resilientindicator contact means for engagement with indicator contact regions onthe heater.

A control of the invention may be integrated with an electricalconnector for the heater, preferably a cordless electrical connector andmost preferably one which engages with a base unit connector in anyrelative angular orientation. Such a connector is disclosed inWO95/08204, to which further reference may be made for constructionaldetails of such a connector.

Preferably the control comprises a moulded body to which the controlcomponents are mounted. In the preferred embodiment one or more of thecontact mounting leaf springs is joined directly to a respectiveterminal of the connector.

From a yet further broad aspect the invention provides an integratedelectrical connector and control unit for a thick film electric heatercomprising an electrical connector having a connector body housingterminals for engagement with terminals in a complementary connectorand, mounted on top of said connector body, and extending laterallytherefrom, a leaf spring mounting a contact for making electricalconnection to the heater, and a bimetallic actuator for making goodthermal contact with the heater.

Most preferably the bimetallic actuator is spring mounted, and mostpreferably the contact and the bimetallic actuators are provided oncommon spring means as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Some preferred embodiment of the invention will now be described by wayof example only, with reference to the accompanying drawings in which:

FIG. 1 is a schematic sectional view showing a control unit inaccordance with the invention mounted with a thick film heater in thebottom of a liquid heating vessel;

FIG. 2 is a plan view of the thick film heater shown in FIG. 1;

FIG. 3 is a plan view of the control unit shown in FIG. 1 looking at theface of the control unit which in use will engage the heater;

FIG. 4 is a plan view of the control unit of FIG. 2 and the heater ofFIG. 3 assembled together, but with the details of the control visibleto assist in understanding how the operative parts of the heater and thecontrol are arranged relative to each other;

FIG. 5 is a plan view of a subassembly of the control unit of FIG. 2;

FIG. 6 is a side view of the subassembly of FIG. 5;

FIG. 7 is a side elevation of other components of the control unit;

FIG. 8 is a first perspective view of a second embodiment of theinvention;

FIG. 9 is a second perspective view of the embodiment of FIG. 8;

FIG. 10 is a section along line I—I of FIG. 8.

FIG. 11 is a top plan view of a further embodiment of the invention;

FIG. 12 is a bottom plan view of FIG. 11;

FIG. 13 is a perspective view of the embodiment of FIG. 11;

FIG. 14 is a perspective view of a yet further embodiment of theinvention; and

FIG. 15 is a detail of a modification of the embodiment of FIG. 14.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, a water heating vessel 2 is provided with athick film plate heater 4 and a thermally sensitive control unit 6.

The vessel 2, for example a hot water jug comprises a plastics body 8having an aperture 10 formed in the base part 12 thereof. The base part12 is formed with a skirt 14, three internally threaded mounting bosses16, a radial sealing flange 18 and a depending sealing flange 20.

The thick film plate heater 4, which is shown in greater detail in FIG.2 comprises a metallic plate 22 having a flat central portion 24 onwhich is provided an insulating layer 26 and a thick film resistiveheating track 28 in a known manner. A protective overglaze (not shown)is also provided over the heating track in a known manner. The heatingtrack 28 is formed as a series of parallel sections whose ends areinterconnected by bridges 30 of silver or other low resistance material.The respective ends of the track are provided with electrical contacts32, 34, which may, for example be contacts bonded onto the terminal padsof the track 28. The plate 22 is provided with a stiffening rim 36,formed with respective peripheral flanges 38, 40 (FIG. 1). The radiallyextending flange is provided with semi-circular scallops 42 forengagement over the vessel mounting bosses 16 and with slots 44 forlocating with complementary formations on the control unit 6.

The track 28 is provided with a region 46 which is formed by a series ofshort track sections 48. As can be seen in FIG. 4, this region isintended to receive the actuator 68 of an overheat protector 62, and isconfigured to minimise the potential drop in the track across the areaof the actuator 68, thereby discouraging shorting of the track 28 to theactuator 68 through the protective overglaze of the heater.

The heater 4 is clamped in the aperture 10 by the control unit 6. Inparticular, the control unit 6 comprises a tray-like plastics mouldedhousing 50, which has three peripherally arranged bores 52 for receivingthe vessel mounting bosses 16 and mounting screws 54. The inner wall 56of each bore 52 is relieved so as to engage the lower face of theradially extending heater flange 40 and so compress a seal 56 betweenthe flanges 38, 40 of the heater plate and those 18,20 of the vesselbody 12, as the screws 54 are tightened.

Referring now in greater detail to FIGS. 3 to 7, the control unit 6comprises an overheat protector 60 and a thermostatic control 62 whoseconstruction and operation will be described below.

The overheat protector 60 comprises a generally U-shaped leaf spring 64.One limb 66 of the leaf spring 64 mounts a snap acting bimetallicactuator 68 of the kind described in GB 1542252. The construction andoperation of such actuators is extremely well known in the art and neednot be described further here. In particular, the tongue 70 of theactuator 68 is received in a raised slot 72 formed in the limb 66. Ascan be seen from FIGS. 1 and 6, the limb 66 is not planar, but is bentdownwardly towards its free end 74 so as to facilitate proper engagementof the actuator 68 with the heater 4, with the periphery of the actuator68 engaging the heater.

The other limb 76 of the leaf spring member 64 mounts a silver contact78 on its free end 80. In use this contact 78 makes electricalconnection to the track contact 32. The free end 80 of the limb 76extends inwardly toward the other limb 66 such that the contact 78 isgenerally aligned on the axis 82 of the leg 66. A tab 84 is folded upout of the limb 76 for positioning closely under a movable peripheralpart 86 of the actuator 68.

The cross limb 90 of the leaf spring member 64 is formed with a deepbend 92 for location within a mounting block 94 provided in the controlhousing 50. The end of the cross member 90 is formed as a spade terminal96 for connection to a mating terminal 98 provided on one wire 100 of apower lead 102, suitably clamped by clamps 104. The clamps 104 are atleast partly formed in a cover moulding 106 which is screwed onto thebottom of the moulding 50 via countersunk bores 108.

It will be seen from FIGS. 1 and 3 that in use, the actuator 68 will bebiased against the region 46 of the heater track 28 by the leg 66 of theleaf spring member 64. The biasing force is typically in the region of25 g, which is sufficient to ensure and maintain good thermal contactwith the heater 4 even with thermal distortion of the heater 4. Thecontact 78 is also biased against the heater contact 34 by the springlimb 76, typically with a spring force of about 10 g.

In the event that the heater overheats, in say the event of the vesselboiling dry or being switched on without containing water, the actuator68 will operate, typically at a temperature of 135-150° C. As it doesso, it reverses its curvature, causing the peripheral portion 86 to moveinto contact with the tab 84 of the contact mounting limb 76, causingthe limb 76 to deflect downwardly and open the contact pair 32, 78,thereby interrupting the supply of energy to the heater. When the heaterhas cooled sufficiently, the actuator 68 will reset, thereby allowingthe contacts 34, 78 to close, so as once again to supply energy to theheater 4. This cycle of operation will continue until the vessel isswitched off or more water is added to the vessel.

Turning now to the thermostatic control 62, this comprises a generallyplanar creep action bimetallic actuator 110 mounted at one end 112 in amounting block 114 provided in the control housing 50. That end 112 ofthe actuator is also formed with a spade connector 116 for connection toa connector 118 provided on the end of the other wire 120 of the powerlead 102. The free end 122 of the actuator 110 mounts a contact member110 which in use makes electrical connection to the track contact 34.

The actuator 110 is urged towards the heater 4 in use by a bent leafspring 126 which is mounted at one end 128 in a mounting block 130provided in the control housing 50. The free end 132 of the leaf spring126 abuts the underside of the actuator 110. The leaf spring 126 isdeflected into greater or lesser engagement with the actuator 110 bymeans of a rotary cam 134 mounted on the end of a control shaft 136received in journals 138 provided in the control housing 50, and whichengages the bent portion 140 of the leaf spring 126. The shaft 136extends through a slot formed in the outer wall 142 of the controlhousing for receiving a control knob (not shown)

In the condition shown in FIG. 7, the cam 134 is in an ‘off’ position,with the leaf spring 126 not pressing substantially on the actuator 110.In this position, the contacts 124, 34 are open, and the heaterdeenergised. However, as the cam 134 is rotated in an anticlockwisedirection (in the sense of FIG. 7), the cam surface 144 will deflect thespring 126 and urge it resiliently into contact with the actuator 110.The first step movement of the cam 134 will cause the actuator contact124 to move into contact with the heater contact 34, thereby allowingenergy to be supplied to the heater. As the cam 134 is rotated further,the leaf spring 126 urges the actuator 110 further towards the heater 4.As the liquid in the vessel heats, heat will be transferred to theactuator 110, which as it heats will try to deflect away from the heater4 to open the contacts 34,124. However, to do this, the actuator 110must overcome the spring force, and the higher the spring force, thehigher the actuator temperature must be to overcome the spring force.When the requisite temperature has been reached, the contacts 34,124will open and the electrical supply to the heater be interrupted. As theliquid in the vessel cools, so will the actuator 110, and eventually itwill deflect back sufficiently far to close the contacts 34,124 onceagain thereby reconnecting the electrical supply. In tests conducted ona 110V, 750 W heater, it has been found that once the liquid in thevessel is at temperature, the control will typically cycle at about 0.5s on, 10 s off, the rapid switch off being due to the heating effect ofthe current (7A) passing through the actuator, rather than the rise intemperature of the liquid in the vessel.

The control settings to obtain a desired liquid temperature can bedetermined empirically, but it will be seen that temperatures up to andincluding boiling can be achieved with the thermostatic controldescribed.

It will be appreciated that the overheat protection control 60 andthermostatic control 62 described above may be employed independently.However, when combined in the manner described they provide a versatilebut simple control structure.

With reference to FIGS. 8 to 10, a further embodiment of the inventionis described, again for use with a thick film plate heater, similar tothat shown in the earlier embodiment.

The control unit 200 shown in FIGS. 8 to 10 is a double pole overheatprotector, comprising first and second overheat protectors 202,204mounted on a tray-like moulded plastics housing 206. The protectors arearranged in the respective poles of the electrical supply to the heater.The housing 206 is moulded with mounting bores 208 for receiving screws,studs, or the like (not shown) which mount the control unit 200 to theheater or to a support for the heater.

Each overheat protector 202,204 has substantially the same constructionas the protector 60 of the first embodiment described above. Thus eachprotector comprises a generally U-shaped leaf spring 212, one limb 214of which mounts a snap acting bimetallic actuator 216 of the kinddescribed in GB 1542252. Each actuator 216 has the same nominaloperating temperature typically 135°-150° C. As in the earlierembodiment, the limb 214 is not planar, but is bent at its free end soas to facilitate proper engagement of the actuator 216 with the heater,to ensure good thermal contact between the actuator and the heater.

The other limb 218 of each leaf spring member 212 mounts a silvercontact 220 which, in use, makes electrical connection to a respectivetrack contact 221. A tab 222 is folded up from the limb 218 forpositioning closely under a movable peripheral part of the actuator 220.

The cross limb 224 of each leaf spring member 212 is formed as shown forlocation within mounting clips 226 provided in the moulding 206. The endof each cross member 224 is formed as a spade terminal 228 forconnection to a mating terminal provided on a power supply lead (notshown).

The contact mounting limb 218 of each leaf spring member 212 is formedwith an extension 230, which engages, with clearance, in slots 232formed in one end of a trip lever 234. As can be seen most clearly fromFIG. 10, the trip lever 234 is mounted to the underside of the moulding206. In particular the trip lever 234 is provided with laterally spacedupstanding pillars 236 having projecting knife edges 238 which engage ingrooves 240 provided in projecting parts 242 of the moulding 206. Theother end of the trip lever 234 is provided with a groove 244 whichreceives one end of a C spring 246. The other end 250 of the C spring246 is received in a groove 248 provided in the moulding 206.

The end 250 of the trip lever 234 is provided with a pair of mountinglugs 252 which receive a knob (not shown) which in use extends to aposition where it can be manipulated by a user.

The trip lever mechanism above acts to prevent cycling of the controlduring operation and also as an on-off mechanism, as will now bediscussed further in the description of the mode of operation of thecontrol unit.

In use, each respective actuators 216 will be biased against the heaterby the leg 214 of the respective leaf spring member 212 and the contacts220 biased against the heater by the spring limbs 218. During normaloperation of the heater, the respective contacts pairs 220,221 remainclosed and power is supplied to the heater. In the event that the heateroverheats, one or other of the actuators 216 will reach its operatingtemperature, reverse its curvature with a snap action, and cause thelimb 218 to deflect to lift the contact 220 from the contact 221 therebyinterrupting the supply of energy to the heater.

Movement of the limb 218 also causes the extension 230 thereof to pickup the trip lever 234 via one wall of the respective slot 232. Thismovement causes the trip lever 234 to move over centre with respect toits C-spring mounting, the C-spring then acting to move the trip lever234 to a position in which the other walls of the respective slots 232pick up both extensions 230, and moves both respective limbs 218 to anopen position in which both sets of contacts are opened. Power cannot bere-supplied to the heater until the trip lever 234 is re-set by a userpushing it over-centre in the opposite direction thereby allowing thecontacts to close once again.

During normal operation of the heater, should a user wish to switch theheater off, he or she may move the operating knob from its “on” positionso that the side of the slot 232 picks up the extensions 230 of bothprotectors 202,204, as above, and lifts the contacts 220 from the heatercontacts 221.

Thus this embodiment provides a simple double pole protector which canalso serve as an on-off switch for the heater. Also, the independentmounting of the actuators allows each to be individually biased intooptimal thermal contact with the heater.

A further embodiment of the invention will now be described withreference to FIGS. 11 to 13. These Figures show an integrated thermallysensitive control and electrical connector unit for use with a thickfilm heater in a water heating vessel such as a kettle or hot water jug.

The unit 300 comprises a connector moulding 302 which mounts on itsupper surface 304 a mounting plate 306 and two actuator/contactassemblies 308,308′. The connector moulding 302 houses electricalterminals arranged in the same configuration as those disclosed in theApplicants' earlier published application WO95/08024 as used in itscommercially available P72 connector. As will be familiar to thoseskilled in the art, these terminals comprise an annular earth terminal310, an annular neutral terminal 312 and a line pin 314, allconcentrically arranged.

Each actuator/contact assemblies 308, 308 is constructed substantiallythe same in principle as the assemblies described in the earlierembodiments, the actuator 316,316′ and contact 318,318′ being mounted onthe respective limbs of a generally U-shaped leaf spring member320,320′. The neutral actuator/contact assembly 308′ is push fitted overa spigot 3221 formed on the moulding 302 and is directly electricallyconnected to the annular neutral terminal 312 of the connector by virtueof receiving tabs 324′ which extend from the upper end of the annularneutral terminal 312.

The line side assembly 308 is also mounted on top of the connectormoulding over a spigot 322 and is connected to a tab 328 intended toreceive a connector of a separate steam switch (not shown). A furthertab 330 which is connected to the other side of the steam switch isconnected to the line pin 314 by a conductor, not shown.

The mounting plate 306 is attached to the connector moulding 302 bymeans of tabs 332 which extend upwardly from the earth terminal 310through the plate and which are then bent over. The mounting plate 306has three mounting holes 336 for engagement over mounting studs providedon the heater.

It will also be noted that the connector moulding 302 has twowing-shaped extensions 340. These wings 340 are open at top and bottomand serve merely to facilitate handling of the control unit and preventaccidental damage to the actuator/contact assemblies 308,308′.

It will be appreciated that if in use when the unit 300 is mounted overthe studs provided on the heater the actuators 316,316′ will be broughtinto good thermal contact with a portion of the heater, and the contacts318,318′ will engage with respective contacts on the heater track.Accordingly, should the heater overheat locally, one or other of theactuators 316,316′ will operate, thereby lifting the associated contact318,318′ from the heater in the manner previously discussed therebydisconnecting the power supply to the heater.

A yet further embodiment of the invention will now be described asreference to FIG. 14. This embodiment discloses an adaptation of theunit of FIGS. 11 to 13 intended to incorporate a steam sensitiveactuator. To this end the unit 400 comprises a moulding 402 generallysimilar to the moulding 302 of the earlier embodiment, but including anextension 404 for mounting a spring loaded, pivotally mounted trip lever406 which is connected to a control knob, (not shown).

The principle of operation of the trip lever 466 is similar to that ofthe arrangement disclosed in FIGS. 8 to 10. To this end it has, at oneend 410 two arms associated with respective actuator/contact assemblies416,418. Each arm has 412,414, has a portion 421 which extends over thecontact mounting leaf spring 420 and another 422 which extends under it.As in the embodiment of FIG. 8, the actuator will operate initially tomove the contact spring 420 which then picks up the arm portion 422 ofthe trip lever 406 which then moves the trip lever 406 over centre, sothat the other arm portion 421 will then engage the contact mountingleaf spring 420 to move it fully away from the underlying heater. Thecontact of the other assembly 416 will also be opened by the trip lever406, meaning that operation of just one actuator will cause double poleswitching.

The moulding extension 404 houses a bimetallic actuator (not shown)which is subjected to steam produced in the liquid heating vessel, whichis directed onto the actuator via a steam opening 430 in a mannersimilar to that disclosed in WO95/34187. When the actuator operates, itwill cause the trip lever 406 to move over centre and thus cause thecontact carrying leaf spring 420 of both assemblies 416,418 to be pickedup by the overlying portions 421 of the respective arms 412,414 and somove the contact thereon away from the heater track.

When the trip arm is reset by a user, then the contact mounting leafspring 420 will be able to return to its original position under its ownresilience.

A further feature of the embodiment of FIG. 14 is that a pair of leafsprings 450 extend from the unit for engagement with contact locationsprovided on the heater. These leaf springs are connected, at the otherside of the control by suitable connectors, to a neon indicator whichindicates when the track is being energised. In the event that the powerto the track is interrupted by operation of the steam or overheatactuators, no power will be supplied to the heater, which means that theneon, whose only source of power is from the heater, will beextinguished.

In the modification of the embodiment of FIG. 14 shown in FIG. 15, eachside of the trip lever end 450 has two arms 452, 454, both of which arearranged under the bimetallic actuator 456. The arm 452 has a rib 458which is positioned under the actuator 456, and the other arm 454extends over the leaf spring 460 mounting the contact 462. In thisarrangement the actuator 456 will, after operation, act initially on therib 458 so as to start pivoting the trip lever over centre. Thatmovement will cause the second arm 454 to pick up the leaf spring 460and so move the contact 462 out of contact with the heater track. Thisarrangement has the advantage that it makes maximum use of the movementand force generated by the actuator 456. Of course, it may be possiblethat the actuator 456 will still impact on the spring 460, depending onthe particular position of the spring relative to the actuator.

It will be appreciated that the invention may be applied to a wholerange of products such as liquid heating vessels, such as kettles andjugs, coffee makers, deep fat fryers and so on.

Also, it will be appreciated that various changes may be made to theembodiments described above without departing from the scope of theinvention. For example, while the second embodiment has been describedas double pole overheat protector, one of the actuators may be chosen tohave an operating temperature such that it acts as a simple thermostat,cycling the power supply to the heater. In that case, no trip levermechanism is associated with the actuator. However, the other actuatormay still act as an overheat protector, being chosen to operate at ahigher temperature, and if desirable, a trip lever mechanism may stillbe associated with that actuator. That trip lever may, however bearranged to open the set of contacts associated with the cyclingthermostat, thereby providing a double pole cut-out.

What is claimed is:
 1. A thermally sensitive control for a thick filmelectric heater comprising a bimetallic actuator arranged for makinggood thermal contact with the heater, and a movable electrical contactfor engagement with a contact provided on the heater, said movablecontact being operatively coupled to the bimetallic actuator such thatin use upon operation of the actuator, it moves out of engagement withthe heater contact to interrupt the electrical supply to the heater,said bimetallic actuator and said movable contact being mounted oncommon leaf spring means which act in use resiliently to bias theactuator into thermal contact with the heater and the movable contactinto electrical contact therewith.
 2. A control as claimed in claim 1wherein said leaf spring means comprises a leaf spring having at leasttwo connected limbs mounting respectively the actuator and contact.
 3. Acontrol as claimed in claim 2 wherein said leaf spring member isgenerally U-shaped, with the contact and actuator mounted on therespective side limbs of the U.
 4. A control as claimed in claim 2 or 3wherein the limb mounting the contact member is arranged to overlap amovable portion of the actuator.
 5. A control as claimed in claim 4wherein the contact carrying limb comprises a folded up tab forengagement with the actuator.
 6. A control as claimed in claim 1 whereinthe actuator is a snap acting actuator.
 7. A control as claimed in claim1 wherein said leaf spring member is provided with a terminal forconnection to an external power supply.
 8. A control as claimed in claim1 wherein the leaf spring member is mounted in a control housing formounting to the vessel or heater.
 9. A control as claimed in claim 1wherein said actuator causes movement of a spring loaded trip leverwhich acts to keep the contact open until reset.
 10. A control asclaimed in claim 1 wherein said control constitutes an overheatprotection means for the heater, operable in the event that the heateroverheats.
 11. A control as claimed in claim 10 comprising a pair ofactuators mounted on respective spring means.
 12. A control as claimedin claim 10 further comprising thermostatic control means for heatingliquid in the vessel to a desired temperature.
 13. A control as claimedin claim 12 wherein said thermostatic control comprises a creep actionbimetallic actuator mounting an electrical contact which in use makescontact with a contact provided on the heater, such that in use whenliquid in the vessel reaches a predetermined temperature, the actuatordeflects to such an extent to lift the actuator contact from the heatercontact.
 14. A control as claimed in claim 13 further comprising meansfor varying the operating temperature of the thermostatic control. 15.An assembly for use in a thermally sensitive control for a thick filmelectric heater, the assembly comprising a snap acting bimetallicactuator, and a movable electrical contact, said bimetallic actuator andsaid contact being mounted on common leaf spring means which in use actto resiliently to bias the actuator into thermal contact with the heaterand the movable contact into electrical contact therewith.
 16. Anassembly as claimed in claim 15 wherein said leaf spring means comprisesa leaf spring having at least two connected limbs mounting respectivelythe actuator and contact.
 17. An assembly as claimed in claim 16 whereinsaid leaf spring member is generally U-shaped, with the contact andactuator mounted on the respective side limbs of the U.
 18. An assemblyas claimed in claim 16 the limb mounting the contact member is arrangedto overlap a movable portion of the actuator.
 19. An assembly as claimedin claim 18 wherein the contact carrying limb comprises a upwardlyfolded tab for engagement with the actuator.
 20. An assembly as claimedin claim 17 wherein said leaf spring means is provided with a terminalfor connection to an external power supply.
 21. A thermally sensitivecontrol for a thick film electric heater comprising a bimetallicactuator arranged for making good thermal contact with the heater, and amovable electrical contact for engagement with a contact provided on theheater, said movable contact being operatively coupled to the bimetallicactuator such that in use upon operation of the actuator, it moves outof engagement with the heater contact to interrupt the electrical supplyto the heater, said bimetallic actuator and said movable contact beingmounted on commonly mounted leaf spring means which act in useresiliently to bias the actuator into thermal contact with the heaterand the movable contact into electrical contact therewith.
 22. Anintegrated electrical connector and control unit for a thick filmelectric heater comprising an electrical connector having a connectorbody housing terminals for engagement with terminals in a complementaryconnector and, mounted on top of said connector body, and extendinglaterally therefrom, a leaf spring mounting a contact for makingelectrical connection to the heater, and a bimetallic actuator formaking good thermal contact with the heater.
 23. A unit as claimed inclaim 22 wherein said bimetallic actuator is spring mounted.
 24. A unitas claimed in claim 23 wherein said actuator and said contact aremounted on common spring means.
 25. An assembly as claimed in claim 17wherein the limb mounting the contact member is arranged to overlap amovable portion of the actuator.
 26. An assembly as claimed in claim 25wherein the contact carrying limb comprises a upwardly folded tab forengagement with the actuator.